A negative electrode for nonaqueous electrolyte secondary batteries is generally produced by mixing particles of an active material formed from a material into which lithium ions can be inserted by charging, with a binder, a conductive material, and a solvent, applying the mixture thus obtained on the surface of a current collector, drying the mixture to form a coating film, and further subjecting the coating film to press processing.
In recent years, along with the development in applications such as electric vehicles and smart phones, there is an increasing demand for capacity increase and lengthening of the service life of batteries. Currently, most of the negative electrodes of commercially available batteries use graphite as the negative electrode active material; however, this active material has already reached the theoretical limit in terms of capacity, and it is necessary to develop new negative electrode active materials. One of the promising candidates thereof is active materials containing silicon (also referred to as “silicon-based active materials”). The silicon-based active materials have a potential that the capacity per mass is 5 to 10 times that of graphite. Therefore, the silicon-based active materials are materials that are particularly drawing attention as negative electrode active materials for batteries.
Various new proposals regarding these kinds of silicon-based active materials are made from various aspects.
For example, it has been proposed that the electron conductivity between an additive current collector and the active material is imparted by, for example, adding a conductive auxiliary agent, in order to increase the electron conductivity of silicon-based active materials (for example, see Patent Document 1 and Patent Document 2).
Further, it has been proposed that a surface treatment is carried out with a silane coupling agent, in order to enhance cycle characteristics of the silicon-based active material (for example, see Patent Document 3).